Callback function invoked when the library asks application how
many padding bytes are required for the transmission of the
|frame|. The application must choose the total length of payload
including padded bytes in range [frame.hd.length, max_payloadlen],
inclusive. Choosing number not in this range will be treated as
ErrorCode.CALLBACK_FAILURE. Returning `frame.hd.length`
means no padding is added. Returning ErrorCode.CALLBACK_FAILURE will make
Session.send() function immediately return ErrorCode.CALLBACK_FAILURE.
Callback function invoked when the library asks application how many padding bytes are required for the transmission of the |frame|. The application must choose the total length of payload including padded bytes in range [frame.hd.length, max_payloadlen], inclusive. Choosing number not in this range will be treated as ErrorCode.CALLBACK_FAILURE. Returning `frame.hd.length` means no padding is added. Returning ErrorCode.CALLBACK_FAILURE will make Session.send() function immediately return ErrorCode.CALLBACK_FAILURE.